While excessive weight gain during pregnancy has been deemed unhealthy for both mother and baby, fetuses can be exposed to higher levels of fat-soluble chemicals in women who don’t gain enough weight during pregnancy, new research finds.

Chemicals, including the highly toxic DDT (which has been banned for more than 40 years) and other persistent organic pollutants can wind up being released into the mother’s bloodstream—and thus into her unborn baby increasing the risk of developmental disorders and delayed learning as well as reduced immune function and hormonal issues.

“The babies’ exposure to all the chemicals decreased as pregnancy weight gain increased,” EHN reports. “However, when the scientists accounted for other factors known to influence concentrations – such as a mother’s age and how much fish she ate – the association was significant for only two chemicals: a byproduct of the insecticide lindane called beta-HCH and a DDT metabolite.”

For every pound of pregnancy weight gain, a 0.75 percent decrease in DDE in the cord blood and a 1.4 percent decrease in beta-HCH were noted. “For the average woman who gained about 31 pounds during pregnancy, that’s about a 22 percent decrease in DDE and a 42 percent decrease in beta-HCH,” EHN explained.

During the third trimester, pregnant women who do not have adequate body fat could lose fat stores as the baby grows rapidly. “This mobilization effect to meet the fetal demand may trigger the release of POPs to the bloodstream, where they may become available and cross the placenta barrier,” the researchers said.

Both the U.S. Institute of Medicine and the World Health Organization recommend women gain between 25 and 35 pounds during pregnancy.